1. Field of the Invention
The present invention relates to a power module board and a power module using the board. More specifically, the present invention relates to a power module board and a power module using the board, which board is used as a radiator board for a power module (power conversion device) such as a machine tool with an electronic control device controlling a motor, an electric car, an electric train or other device in which a large power flow is controlled by a semiconductor device.
2. Description of the Background Art
In order to drive a motor of a machine tool, an electric car, an electric train or the like, a power module performing DC-AC conversion and DC-DC conversion has been employed. The power module includes an IGBT (Insulated Gate Bipolar Transistor) unit for performing DC-AC conversion, a counter flow preventing capacitor, a control circuit unit and so on.
A large current is taken out from a front surface and a rear surface of the IGBT chip (here it is assumed that the chip includes a semiconductor device). Therefore, high electric insulation is required of a board on which the IGBT chip is fixed. Further, as an electrode for taking out current from the rear surface is formed on an electrically insulating board, a conductor layer must be formed on the electrically insulating board.
Generally, a thin film mainly consisting of copper (for example, a layer formed of copper foil) is used as the conductor layer, which thin layer is provided between IGBT chip and ceramic base plate, with an intervening layer therebetween.
Further, chip temperature of the IGBT chip increases because of heat for bonding generated at the time of mounting, and because of heat generated by the semiconductor device in operation, as a large current is controlled. Therefore, the board on which the IGBT chip is fixed and the entire board including peripheral members must have high radiation characteristic. If radiation from the board is insufficient, the temperature of the semiconductor device becomes too high to enable current control, and in the worst case, the semiconductor device may be broken.
FIG. 22 is a cross section of a conventional power module. In the conventional power module, IGBT chip 6 is fixed by solder on a copper layer 8 on a ceramic base plate 1 as an insulating base plate, and the ceramic base plate 1 is fixed by solder on a metal base 12. In FIG. 22, control circuits are not shown.
Referring to the figure, on a heat radiating plate 5 formed of a metal having high thermal conductivity such as copper or aluminum, base 12 is placed. Base 12 is fixed on heat radiating plate 5 by means of a bolt 4. Though not shown, at an interface between heat radiating plate 5 and base 12, a thin layer of silicon oil compound or the like is formed to reduce thermal resistance at the interface.
On base 12, a resin case 10 encapsulating the current control unit is fixed by means of an adhesive, for example. On base 12 in case 10, ceramic base plate 1 on which a control circuit part is mounted is fixed by solder. More specifically, a control circuit having the intervening layer and conductive layer 8 mainly consisting of copper stacked with each other is formed, on ceramic base plate 1 with one end connected to an external circuit through an electrode 9.
Circuit components such as semiconductor device 6 and diode 7 are soldered on conductive layer 8. The space of the encapsulating case 10 is filled with silicon gel, for example. Here, wire bonding between the semiconductor device, diode and the like are not shown.
Base 12 is formed of a metal such as copper or aluminum, or a composite material containing a higher proportional amount of metal such as copper-tungsten or aluminum-silicon carbide. Aluminum nitride ceramics is used for ceramic base plate 1, as it requires electrical insulation and high thermal conductivity.
For a power module having the IGBT chip of large capacity and hence large amount of heat build up, a structural member improving radiation such as a water cooling radiator or an air cooling fan is arranged inside or below heat radiating plate 5.
With respect to the structure of conductive layer 8, as disclosed in Japanese Patent Laying-Open No. 9-275166, various structures of the intervening layer have been proposed to relax thermal stress between the copper conductive layer and the ceramic base plate formed of aluminum nitride ceramics, for example, which have very different coefficients of thermal expansion.
Not only one unit of the combination of ceramic base plate-intervening layer-conductive layer but two or more units of this combination in this order may be stacked to form a module.
For example, at the portion of conductive layer 8 shown in FIG. 22, for example on the ceramic base plate, two or more units may be stacked, such as intervening layer-conductive layer-intervening layer-ceramic layer-intervening layer-conductive layer-intervening layer- . . . .
As described above, in the conventional power module, IGBT chip 6 as a semiconductor device is fixed on base 12 with a ceramic base plate 1 interposed. In order to simplify this structure to reduce cost, it may be possible to directly fix the electrically insulating ceramic base plate having high thermal conductivity such as aluminum nitride ceramic on the box body, for example, without using base 12.
In this case, two different configurations are possible. Namely, the configuration of FIG. 23A in which ceramic base plate 1 is used as an integral body on which a plurality of IGBT chips 6 are fixed, and a configuration of FIG. 23B in which ceramic base plate 1 is divided (in this example, divided into two).
In either of the configurations of FIGS. 23A and 23B, when ceramic base plate 1 is directly fixed by a bolt 4 or the like or by interposing a washer 16 as shown in FIGS. 23A and 23B, cracks generate at portions where stress concentrates, at the fixing portion, as ceramic base plate 1 has lower mechanical strength, as compared with when base 12 (see FIG. 22) is used for fixing.
When IGBT chip 6 as the semiconductor device operates and temperature increases, greater stress generates at the fixing portion, resulting in cracks, because of the difference in the coefficients of thermal expansion between conductive layer 8 and ceramic base plate 1 and between heat radiating plate 5 and ceramic base plate 1.
Further, as fixing on radiating plate 5 is done without using solder, radiation must be performed with high efficiency. In order to improve thermal conductivity of base 12 and heat radiating plate 5, it is possible to reduce thermal resistance at the interface between heat radiating plate 5 and ceramic base plate 1 by applying silicon oil compound. Aluminum nitride ceramics or silicon nitride ceramics used as the material of ceramic base plate 1, however, has poor wetting property with silicon oil compound.
Further, at the fixing portion where ceramic base plate 1 is fixed on heat radiating plate 5, heat resistance must be decreased. When a washer is used as is conventionally common, the problem of higher heat resistance at the fixing portion results. When the conventional base containing metal is used, increase in heat resistance can be suppressed even when there are small recesses or protrusions on the surface of heat radiating plate 5, as base 12 deforms to reduce air layer at the interface therebetween. When the ceramic base plate is used as the base, there arises a problem that cracks generate from the small recesses or protrusions.
When the configurations of FIG. 23A and 23B are compared, the area of the expensive ceramic base plate 1 is relatively small in FIG. 23B as compared with FIG. 23A, and therefore the cost for the raw material can be reduced. At the time of mounting, thermal stress can be dispersed and relaxed. This effect is promoted when ceramic base plate 1 is divided into larger number. Further, it is expected in the future that the ceramic base plate 1 comes to be thinner, and that the degree of integration of IGBT chip 6 is increased, resulting in larger amount of heat. The configuration of FIG. 23B is considered to be more advantageous for apparatuses of larger size. The configuration of FIG. 23B, however, requires higher cost for fixing, as the number of units to be mounted and fixed increases, as compared with the configuration of FIG. 23A.
Further, in order to reduce the area of ceramic base plate 1, use of a fixing jig 3 such as shown in FIG. 24 is more advantageous than direct fixing by means of bolt 4, as the portion of the base plate in which the bolt hole is formed can be saved. When only a pair of sides of ceramic base plate 1 are fixed as shown in FIG. 24, ceramic base plate 1 may highly possibly be displaced when a force parallel to the sides is applied. In view of safety against impact, this is a significant problem especially in a car or other vehicle.